Living opals nano-manufactured in common seaweed, researchers discover

Image credit: Dreamstime

By Hilary Lamb

Published Wednesday, April 18, 2018

Researchers based at the University of Bristol have discovered a type of opal grown by common seaweed, which self-assembles nanostructures of oil to create a shimmering effect previously thought unique to opals.

Opal is a type of silica with ordered internal structures which cause it to diffract light, demonstrating iridescence. During the Middle Ages, opal was considered auspicious and even thought to enable the bearer to become invisible when wrapped in a bay leaf. Until now, this shimmering display of colours was thought to be unique to the gemstone.

Now, however, a new type of opal has been discovered in a common European seaweed, brown algae seaweed, which self-assembles nanostructures to control how light is reflected from its cells. This creates a blue-green sheen on its surface similar to that of opal stones.

The team of nano-optics experts found that the iridescent sheen (‘rainbow wrack’) that appears on the seaweed comes about due to the growth of a nanostructure which controls the amount of light being reflected from its cells containing chloroplasts, in which photosynthesis takes place.

University of Bristol

Image credit: University of Bristol

These tiny structures arise from the tight and ordered packing of nanospheres, which reflect incident white light at a range of angles. In opal gemstones, these nanospheres are of glass and are formed in conditions deep within the Earth. In opalescent seaweed, however, these nanospheres are made from small droplets of oil. Seaweed cells containing chloroplasts self-assemble the oil droplets into a regular structure in a matter of hours.

“The formation of opals from oil droplets is a completely new discovery. If nanotechnologists were able to understand and mimic the dynamic properties of this seaweed opal, we may in the future have biodegradable, switchable display technology that may be used in packaging or very efficient, low-cost solar cells,” said Dr Martin Lopez-Garcia, who was involved in the research at the University of Bristol.

The researchers found that this type of seaweed was even capable of stopping and starting the growth of these nanostructures, resulting in living opals which react to changing sunlight in rockpools. When illuminated, the seaweed’s iridescence vanishes, although it reappears under low-light conditions. Imaging the seaweed demonstrated the researchers’ suspicions that the seaweed was “switching on and off” its self-assembly mechanism: they observed a switch from a chaotic, dull and unreflective structure to an ordered, iridescent structure.

The researchers suggest that this unusual mechanism may have emerged in order to scatter light evenly to all the cell’s chloroplasts, as light levels change throughout the day.

“In the past decade, nanotechnologists have been able to make artificial opals out of similar glass nanospheres. But it seems that the humble seaweed is also able to make such artificial opals in its cells,” said Dr Ruth Oulton, reader in quantum photonics at the University of Bristol.

“So next time you’re rock pooling in the UK during your summer holidays, see if you can find this amazing seaweed with its nano-manufacturing technology.”